The present invention relates to a circuit and a method for operating a radio telephone in spite of a power failure, and more particularly to a circuit and a method for operating a radio telephone branched with a wire telephone by a telephone line even when a power failure occurs during an engaged state in a call.
In general, a wire telephone is operated by a voltage of -48 V served from a switching system, while a radio telephone requires an additional voltage to be supplied, so that the radio telephone has a power supply device for providing an operating voltage. The power supply device supplies only a direct current (dc) voltage to the radio telephone by dropping an alternating current (ac) voltage of 110 V or 220 V to a given voltage and rectifying the dropped voltage. The dc voltage from the power supply device is used as an operating voltage source for all of the circuits in a fixed device of the radio telephone and as a charging and operating voltage source of a portable device of the radio telephone. An adapter is generally used for the power supply device.
A conventional radio telephone operated by a dc voltage from an adapter, however, has a problem that a telephone line engaged in a call is cut off when a power failure occurs during the call.
To solve the above problem, there is a method of installing a charging device like a storage battery or a capacitor in a fixed device or an adapter of a radio telephone in order to supply a dc voltage to the radio telephone when a power failure occurs, the charging device being charged while power is normally supplied. But this method also has a problem that the volume and the cost of the radio telephone are increased.
Another method is branching a wire telephone with a radio telephone by a telephone line so that a call can be made through the wire telephone in case of a power failure of the radio telephone. However if a power failure occurs while a party is engaged in a call through the radio telephone, the call is not switched into the wire telephone but cut off immediately.